The present invention relates to an avalanche photodiode (APD) used for optical communication, or more in particular, to a high-speed, high-sensitivity superlattice avalanche photodiode.
If an APD is to be increased in speed and decreased in noise, it is necessary to greatly differentiate the ionization rate for electrons and holes from each other. One structure known for improving the ionization rate in this way is a superlattice APD. In the prior art, an effective well width within a multiplication area of a superlattice APD has been considered to be in a comparatively wide range (say, from 20 to 50 nm) where the quantum effect is not significantly exhibited. (See, for exmaple, IEEE, Trans. on Electron Devices, ED-33, No. 10 (1986), pp 1502-1510.) This is by reason of two facts, one that the ionization rate threshold energy is increased by the quantum effect, and the other that the band discontinuity value is decreased.